(1) Field of the Invention
This invention relates to two-dimensional radiation detectors suited for use in radiographic and other diagnostic apparatus to detect radiation such as X rays, including two-dimensional positions of incidence thereof.
(2) Description of the Related Art
Various proposals have heretofore been made relating to two-dimensional radiation detectors of the solid-state scan type (see Japanese Patent publications (Unexamined) Nos. 1990-253185, 1991-185865, 1992-206573, 1992-212456 and 1992-212458, for example). To sum up these proposals, a conventional two-dimensional radiation detector may be said to have a construction as shown in FIGS. 1A-1E, 2 and 3.
Referring to FIG. 1A, the conventional detector has a multi-layer structure including a scintilator 11, a translucent electrode film 12, a photoconductive film 13 and a scan switch layer 14. As shown in FIG. 1B, the translucent electrode film 12 is in the form of a uniform plane throughout the entire area thereof. The scan switch layer 14 includes conductors 41 arranged in matrix form and in contact with the photoconductive film 13, row conductors 42 in strip patterns corresponding to the rows of conductors 41, column conductors 43 in strip patterns corresponding to the columns of conductors 41, and a multiplicity of switching elements (FETs) 45 each having the drain connected to one of the matrix conductors 41, the source connected to one of the column conductors 46 and the gate connected to one of the row conductors 42 (see FIGS. 1C, 1D, 1E, 2 and 3). The translucent electrode film 12 receives a predetermined potential from a bias source 44. The row conductors 42 of the scan switch layer 14 are connected to control lines of a drive circuit 15, respectively. The column conductors 46 are connected to signal reading circuits 16 of signal reading lines, respectively.
The scintilator 11 produces light when hit by X rays, and the light is led through the translucent electrode film 12 to the photoconductive film 13, whereby electric charges accumulate on the photoconductive film 13. That is, the scintilator 11 converts a radiographic image into an optical image, and the photoconductive film 13 converts this optical image into an electric charge image. The electric charges on the photoconductive film 13 are read pixel by pixel, each pixel corresponding to each matrix conductor 41, by actuating the FETs 45. Specifically, when the drive circuit 15 supplies a drive signal to one of the row conductors 42, all of the FETs 45 in that row (e.g. row "i") are turned on to cause charge storing currents corresponding to the respective pixels to be retrieved simultaneously from the matrix conductors 41 in row "i" and in the respective columns, through the respective column conductors 46.
In the conventional two-dimensional radiation detector described above, the column conductors 46 are directly connected to the electrodes (sources) of the switching elements (FETs). Consequently, switching noise of the switching elements is transmitted to the sources through stray capacitance between the gate and source to be superimposed on read signal currents, thereby lowering image quality.